Method for making blow molded containers with high length to diameter ratio

ABSTRACT

This invention increases the length-to-diameter ratio of plastic articles made on injection molding machines without using long and relatively thin core rods. This added length is obtained by having the first part of the blowing operation stretch the parison lengthwise before there is any substantial displacement of the parison in a radial direction. At the start of the blowing operation, a tube surrounds and confines radial expansion of the parison, but this tube is withdrawn progressively as the blowing operation continues.

This application is a divisional application of copending applicationU.S. Ser. No. 512,294, filed Oct. 4, 1974 now U.S. Pat. No. 3,936,260.

One of the difficulties in making containers on injection blow moldingmachines is that there are limitations on the length of the containerwith respect to its diameter or cross section. One way to increase thelength-to-diameter ratio is to use longer core rods but this has thedisadvantage that the long core rods do not have the stability ofshorter ones and increasing the cross section of the core rod makes itunusable on conventional machines.

This invention uses a different expedient for increasing thelength-to-diameter ratio without changing the core rods. A novel blowingmold is used which causes the parison to be blown first in a lengthwisedirection and then to the full cross section of the blow mold cavity.This differential blowing action is obtained by having a tube which fitsinto the blow mold cavity from the end opposite that through which thecore rod projects, and with the tube in axial alignment with the corerod and slightly larger than the cross section of the parison.

The tube prevents radial expansion of the parison but permits it toelongate when the blowing operation starts. As the blowing operationcontinues, the tube is withdrawn from the blowing mold and the parisonexpands progressively starting from the neck end of the core rod.

The invention requires a different blowing mold, but different molds arerequired for every different container anyway. The novel mold of thisinvention has simple apparatus for withdrawing the tube from the moldand it can be added to conventional blow molding machines.

Other objects, features and advantages of the invention will appear orbe pointed out as the description proceeds.

In the drawing, forming a part hereof, in which like referencecharacters indicate corresponding parts in all the views:

FIG. 1 is a diagrammatic top plan view showing the kind of injectionblow molding apparatus with which this invention is used;

FIG. 2 is a diagrammatic sectional view showing the novel mold andconfining tube of this invention;

FIGS. 3 and 4 are sectional views taken on the line 3--3 and 4--4,respectively, of FIG. 2;

FIG. 5 is a fragmentary view, similar to FIG. 2, showing the blowing ofthe parison as the tube withdraws from the blowing mold; and

FIG. 6 is a detail sectional view showing a modified form of tube forconfining the parison during the initial part of the blowing operation.

FIG. 1 shows injection blow molding apparatus including an indexing head10 which rotates about a center shaft 12 and which has three faces fromwhich core rods 16 extend. These core rods extend into an injection mold18, a blowing mold 20 and to stripper mechanism 22 when the indexinghead 10 is located in any one of three positions at which it stops aftersuccessive 120° angular movement.

The indexing head may have four faces or even more faces for providingadditional operational stations around its perimeter. Such blow moldingapparatus is well known and no further description of it is necessaryfor a complete understanding of this invention.

FIG. 2 shows a blowing mold 20' into which a core rod 16' extends. Thereis a parison 26 on the core rod 16'. The blowing mold 20' has an uppersection 28 and a lower section 29 which are shown in closed position inFIG. 2. With the mold thus closed, semi-circular recesses 30 in theconfronting faces of the mold sections 28 and 29 grip the core rod.These recesses 30 are also shaped to fit around the neck end of theparison 26 in accordance with conventional practice.

The cavity of the mold 20' is indicated by the reference character 34.At the end of the cavity 34 opposite the end through which the core rod16' extends there is an opening 36 through which a tube 38 extends intothe blow mold cavity 34. This tube 38 is in axial alignment with thecore rod 16' and the inside diameter of the tube 38 is slightly largerthan the outside diameter of the parison 26, so that there is a smallclearance between the parison and the tube 38 when the tube is in theposition shown in FIG. 2.

The tube 38 is movable axially in the opening 36 as a bearing. The tube38 also slides on a shaft 40 which is preferably in a fixed positionwith an end face 42 in position to form, in effect, a part of the endwall of the blow mold cavity 34.

Although the shaft 40 is preferably fixed, FIG. 2 shows the shaft 40connected to a piston 43 which slides in a cylinder 44 and through abearing 46 secured to one end of the cylinder 44. The cylinder 44 isattached to fixed structure 48 so that as long as the piston 43 remainsstationary in the cylinder, the shaft 40 is held in fixed position.

The reason for having the piston 43 and cylinder 44 is to permit theshaft 40 to be advanced toward the left so that it approaches closer tothe end of the parison 26 and one technique for using this invention isto move the shaft 40 toward the right so as to create a reduced pressureahead of the parison 26 to cause more rapid expansion of the parison ina longitudinal direction.

A collar 52 is secured in an adjustable but fixed position on the shaft40 and locked in position by a set screw 54 so that the collar 52strikes against the end face of the bearing 46 when the shaft 40 reachesthe position shown in FIG. 2 with the end face 42 in line with the restof the end wall of the blowing mold cavity 34.

Power means are provided for moving the tube 38 along the shaft 40 so asto shift an end wall 58 toward and from the front wall 60 of the blowmold cavity 34.

The means for moving the tube 38 includes a bracket 62 rigidly securedto the tube 38. A cylinder-and-piston motor 64 has a piston rod 66 whichconnects with the bracket 62 so that reciprocating movement of thepiston rod 66 transmits equal movement to the tube 38. Thecylinder-and-piston motor 64 is representative for means for moving thetube 38 progressively in accordance with the operation of the blowmolding apparatus. FIG. 2 shows valve mechanism 68 for operating themotor 64 as a double acting motor and other control means 70 operate thevalve in accordance with the supply of air to the core rod 16' at theblowing mold 20'.

At the beginning of the blowing operation, a valve 74 at the end of thecore rod opens to permit air to flow from the core rod into contact withthe parison 22. The pressure of this air expands the parison 26longitudinally while the tube 38 prevents the parison from expandingradially. The clearance between the outside of the parison 26 and theinside surface of the tube 38 provides an air cushion so that as theparison 26 begins to expand it can be confined by the tube 38 withouthaving the parison actually come in contact with the outside wall of thetube. This is advantageous so that progressive withdrawal of the tubefrom the mold cavity will not tend to drag on the outside surface of theparison.

At the beginning of the blowing operation, the tube 38 has its end face58 against the front wall 60 of the blowing mold cavity 34. As theblowing operation progresses with the parison 26 expandinglongitudinally toward the end face 42 of the shaft 40, the motor 64starts to operate and to move the tube 38 toward the right so that itwithdraws progressively from the blow mold cavity 34. This movement ofthe tube 38 is preferably delayed until the end of the parison 26reaches, or almost reaches, the face 42 of the shaft 40.

FIG. 5 shows the tube 38 partially withdrawn from the blow mold cavity34. As the end face 58 moves toward the right in FIG. 5, the parison 26is unconfined for progressively greater distances toward the right andthese unconfined portions of the parison 26 expand radially into contactwith the wall of the blow mold cavity 34, as shown in FIG. 5. Thus theparison 26, which has been blown to substantially its full length, nowexpands radially to its full width and thus produces a blown containerof substantially greater length than the core rod 16'.

FIG. 6 shows a modified tube 38'. This tube differs from the tube 38shown in FIGS. 2-5 in that it has a double wall enclosing a chamber 78into which air is supplied with enough pressure to provide an effectiveair cushion between the parison 26 and the inside surface of the tube38'. The air from the chamber 78 discharges through openings 80 in theinner wall of the tube 38'. This provides an effective air cushion foroperations where the blowing time is somewhat longer than with theconstruction shown in FIG. 5. The tube 38', however, operates in thesame way as the tube 38 insofar as its movement into and out of the blowmold cavity are concerned. The tube 38' also has the shaft 40 extendinginto it in the same way as already described in connection with FIGS.2-5.

The preferred embodiment of the invention has been illustrated anddescribed, but some features can be used in different combinationswithout departing from the invention as defined in the claims.

What is claimed is:
 1. A method for injection blow molding a containerhaving a substantially greater length than diameter comprising;(a)injection molding a parison on a core rod; (b) providing a blow moldcomprising a tube being formed with a substantially greater lengthlonger than diameter cavity, said tube being disposed about the parisonand core rod; (c) providing an air flow between the tube and the parisonto maintain a clearance between the parison and the tube; (d) firstblowing the parison to increase the length to diameter ratio of theparison so that said parison length approaches the length of thecontainer being formed; and then (e) final blowing the parison whileprogressively withdrawing the tube with the progressive blowing of theparison to the final diameter of the container.
 2. The method of claim1, wherein the step of blowing the parison further comprisesprogressively blowing the diameter of the container from the top to thebottom of the container.
 3. The method of claim 1, further comprisingproviding an air cushion between the inside surface of the tube and theoutside of the parison.
 4. The method of claim 3, further comprisingcontrolling the pressure of the air in expanding the parison whilecontrolling the pressure of the air cushion.
 5. A method for injectionblow molding a container having a substantially greater length thandiameter comprising;(a) injecting a parison having a neck and a bodyportion on a core rod in an injection mold; (b) transfering the parisonand core rod from an injection mold to a blow mold comprising a tube;(c) inserting the parison and core rod into the tube within the blowmold, said tube being formed with a substantially greater length thandiameter cavity; (d) increasing the length of the parison in the tube byfirst blowing the parison in the tube; while preventing contact betweenthe inside of the tube and the outside of the parison; and then (e)progressively withdrawing the tube while blowing the parison body so asto progressively blow the diameter of the parison from below the neck tothe bottom of the blown container.
 6. The method of claim 5, furthercomprising maintaining a clearance between the inside of the tube andthe outside of the parison.
 7. The method of claim 6, further comprisingproviding an air cushion between the inside surface of the tube and theoutside of the parison.
 8. The method of claim 5, further comprisingterminating the withdrawal of the tube so that the end of the tube is atsubstantially the end of the blow mold cavity and being the end of thetube about flush with an a abutment surface.
 9. The method of claim 8,further comprising timing the withdrawal of the tube in relation tocompletion of the blowing of the parison.
 10. The method of claim 9,further comprising varying the speed of the tube withdrawal in said timerelationship.
 11. The method of claim 5, wherein the step of insertingthe parison and core rod in the tube comprises aligning the parison andcore rods to be substantially coaxial with the tube.